Comparatively Speaking: Number of Molecules in Compounds vs. Compositions

In this basic look at representative chemical structures, Tony O'Lenick compares the number of molecules present in sodium chloride with those present in sodium polymethacrylate. Future features will discuss the accuracy of the structures themselves as well as differences in the distribution of the molecules.

Sodium chloride (Figure 1)  is a compound, whereas sodium polymethacrylate (Figure 2) is a polymeric composition. There is a salient difference between polymeric compositions and compounds. Pure compounds like NaCl are single molecules; they have a specific molecular weight (58.44) and can be referred to as compounds. If one purchases this material from a manufacturer, it will be the same regardless of the manufacturer.

Polymers, on the other hand, are complex oligomeric mixtures, and the value of x (see Figure 2) can vary over a wide range. This means there is not a singular molecular weight but a molecular weight range. These types of materials contain many compounds and are referred to as compositions. The range of the x value can be controlled by process, catalyst and blending, which means there are possible product variations from manufacturer to manufacturer depending on how the product is made.

Number Average and Weight Average Molecular Weight
The number average molecular weight is the total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample. The weight average is more complicated, however, since it is based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do (see Figure 3).

In the case of blends, when two materials with the same INCI name but differing molecular weights are blended, the INCI name of the blend is unchanged (see Figure 4) and the reported molecular weight reported is unchanged; however, the performance very well could change.

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